1) Field of the Invention
The present invention relates to an apparatus that reduces cable television picture signal interference caused by external radio frequency (RF) signals picked up and carried by the shielding of a coaxial cable communicating with a television or video cassette recorder (VCR). The apparatus comprises a pair of RF signal chokes that are formed by wrapping a coaxial cable such as Belden 9308 mini-hardline coaxial cable which offers 100% shielding around a pair of ferrite toroidal cores. The apparatus filters from the coaxial cable shielding the undesired external RF signals that are picked up and transmitted on the shielding to the television or VCR tuner where they interfere with the cable system signals transmitted by the coaxial cable center conductor. The apparatus is provided in a compact housing with input and output connectors that is easily connected between a cable system coaxial cable extending from a wall outlet and a television or VCR receiver by a layperson not having the specialized training of a television technician.
2) Description of the Related Art
Cable television signals are often fuzzy and unclear to the viewer due to interference which is virtually impossible to eliminate. The interference is often caused by secondary, "off-air" radio frequency (RF) signals that are picked up by the shielding of the coaxial cable, which acts as an antenna. Often, the picture interference problems stem from the design of the television of the cable system subscriber, something over which the cable signal supplier has no control. The problem persists because television manufacturers continue to build televisions with tuners that are susceptible to RF signal interference. The television owner does not know what causes the problem, and so doesn't know who to turn to for its solution.
If the shielding of the coaxial cable or the internal shielding of a TV or VCR tuner connected to the coaxial cable is inadequate, the RF interference may be directly picked up and mixed with the cable system's primary signals carried by the center conductor of the coaxial cable. The mixing of the "off-air" RF signals with the cable system's primary signals interferes with the television's or VCR's reception of the primary signals. This is known as the direct pick up ("DPU") problem in the CATV industry. External "off-air" RF signals produced by sources such as local television stations, ham operators and beeper services cause picture interference when they mix with the desired CATV signals on the coaxial cable center conductor. If these external RF signals are conducted along the shielding of the coaxial cable to the television or VCR tuner and are allowed to enter the tuner where they can mix with the desired CATV signals carried on the coaxial cable center conductor, they will cause picture degradation such as ghosting or diagonal lines in the television picture.
Most televisions and video cassette recorders are manufactured with tuners that have small access holes in them. The holes are used in the factory to calibrate the tuner during assembly. However, manufacturers often leave the holes open after the tuner has been calibrated. When a cable system's coaxial cable is connected with a tuner having access holes, the external RF signals conducted on the shielding of the coaxial cable are communicated to the housing of the television tuner. From the housing, the RF signals then pass through the access holes and enter into the tuner where the tuner circuitry is located. Once inside the tuner, these undesired external RF signals mix with the CATV signals carried by the center conductor of the coaxial cable and reduce the quality of the picture produced by the television or VCR tuner.
Various methods and apparatus have been employed in the prior art to reduce external RF signals conducted on the shielding of coaxial cables. However, these methods are not commonly known within the CATV industry, are not easy to implement, and therefore are rarely used within the CATV industry. One method of attenuating RF signals conducted on the shielding of coaxial cables involves packing a cylindrical tube with steel wool, and then running the coaxial cable through the center of the steel wool packed in the tube. The steel wool is lossy and dissipates the RF energy on the cable shield. To increase this device's effectiveness in attenuating RF signals, the tube should be 18 inches long or longer. A disadvantage of this method is that the long tube required to manufacture the apparatus is awkward and unsightly. A cable television subscriber would likely forego using this apparatus to improve his television or VCR reception because it requires the awkward, unsightly tube to be connected along the length of coaxial cable connecting a coaxial cable wall outlet with their television or VCR receiver. The large tube is difficult to conceal and the subscriber would likely be unwilling to have the tube visible in their home.
Another method employed in attenuating external RF signals conducted on the shielding of coaxial cable involves connecting a metal plate between the shielding of the length of coaxial cable communicating a coaxial cable wall outlet with a television or VCR receiver. For VHF television channel 2, the smallest effective metal plate is a 9 foot diameter circle. The required size of the plate decreases linearly with frequency. However, a common cable television subscriber would likely be opposed to having a metal plate of this size connected in the coaxial cable communicating his television or VCR with the coaxial cable wall outlet in his home. Also, the TV owner would need a different sized plate for each channel frequency that he desires to filter.
A third method employed to attenuate external RF signals conducted on the shielding of coaxial cable involves winding the cable on a ferrite toroid. Although the cable extending from a typical wall outlet to a TV or VCR is flexible, no provision is made for securing the toroid adjacent to the TV so as to maximize its effectiveness. Furthermore, as with the previously-described prior art apparatus, a toroid wrapped with a connection cord would be awkward and unsightly, and the coils wound around the toroid would be likely to unwind each time the cable is disconnected from the television.
Besides the disadvantages of the large size and unsightly appearance of the prior art apparatus discussed above, each of the apparatus are also disadvantaged in that they are not commercially available and their existence is not common knowledge of the layperson cable television subscriber. The above-described jerry-built apparatus are generally known to only a few cable television technicians having specialized knowledge of external RF signals and the problems they cause to television reception. At present, for a common cable television subscriber to improve the picture reception of their television or VCR connected with a cable system's coaxial cable, the layperson would have to call a technician into their home and explain the problem to the technician, and then the technician would have to have the requisite knowledge to identify the problem as external RF signal interference. The technician would also have to have the requisite knowledge to remedy the problem by constructing from scratch one of the above-identified RF signal attenuating apparatus. This is unlikely to occur as these apparatus are not widely known in the cable television industry. The layperson cable subscriber would then have to be resigned to put up with the awkward and unsightly signal attenuating apparatus in their home in order to improve his cable television picture reception.
At present, cable television technicians often attempt to solve the problem of RF signal interference by connecting a standard cable converter box to the coaxial cable suppplying signals to the television in the hope that this will solve the problem. However, the converter boxes are often ineffective as shield filters and connecting the box to a cable ready television defeats the cable ready capability of the television.
What is needed to overcome the above-described RF signal television picture interference and the disadvantages associated with known RF signal attenuation apparatus is a commercially available, compact external RF signal filter apparatus that can be readily and easily connected by a layperson between the coaxial cable extending from the wall outlet of the layperson's home and their television or VCR.
It is an object of the present invention to provide a method for an unskilled layperson to attenuate television picture interference caused by secondary "off-air" RF signals carried on the shielding layer of a cable system's coaxial cable.
It is also an object of the present invention to provide an inexpensive compact apparatus that is easily connected by a layperson between a length of coaxial cable leading from a cable system wall socket and a television or VCR receiver, and that filters "off-air" RF signals carried on the shielding layer of the coaxial cable.
A further object of the present invention is to provide an apparatus that filters cable signals that leak out of a television tuner and travel on the coaxial cable shield or leak into the air where they interfere with aircraft navigation and communication, television "off-air" signals and radio communications.
It is also an object of the present invention to provide a filtering apparatus having a simplified construction that enables a layperson having no specialized skills to connect the filter closely adjacent to a television or VCR receiver.